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Phenolic acids interactions with clay minerals: A
spotlight on the adsorption mechanisms of Gallic Acid
onto montmorillonite
Adoum Mahamat Ahmat, Thomas Thiebault, Régis Guégan
To cite this version:
Phenolic acids interactions with clay minerals: a spotlight on the adsorption
mechanisms of Gallic Acid onto Montmorillonite
Adoum Mahamat Ahmat(a,b)1, Thomas Thiebault(c)
, Régis Guégan(b,d) 2
(a) Institut Mines-Telecom Lille-Douai, Department of Civil Engineering and Environment. 764, Boulevard Lahure, 59508 Douai, France.
(b) Institut des Sciences de la Terre d’Orléans ISTO, UMR 7327, University of Orléans, 1A Rue de la Férollerie, 45500 Orléans La source, France.
(c) EPHE, PSL University, UMR 7619 METIS (SU, CNRS, EPHE), 4 place Jussieu, F-75005, Paris, France
(d)
Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.
Abstract
For a better understanding of the preservation of organic matter in clay minerals, the 1
adsorption of a model humic substance, the Gallic Acid (GA), onto a Na-montmorillonite 2
(Na-Mt) was performed in batch situation for various experimental conditions (pH=2, 5, 7) in 3
order to mimic the natural context. The adsorption efficiency and change in the clay mineral 4
were characterized via a set of complementary experimental techniques (Fourier transform 5
infrared spectroscopy, X-ray diffraction, elemental analyses). Adsorption isotherms at the 6
equilibrium were fitted with the models of Langmuir, Freundlich and Dubinin-Radushkevitch 7
allowing one to precisely quantify the adsorption through the derived fitting parameters. From 8
the adsorption data combined with complementary results of the modeled humic-clay 9
complexes, different types of interactional mechanisms were inferred as a function of 10
background acidity: (i) at pH=2 while protonated GA was the preponderant form, anionic GA 11
species can be adsorbed to the Na-Mt surface through electrostatic interaction, leading to the a 12
1
Corresponding author. E-mail address: adoum.mahamat-ahmat@bordeaux-inp.fr (A. Mahamat Ahmat).
*Revised manuscript with no changes marked
slight covering of the clay surface favoring in a second step the GA adsorption by π-π and 13
Van der Waals forces; XRD patterns corroborated via TGA and FT/IR results suggested the 14
actual intercalation of the phenolic acid within the interlayer space; (ii) At pH = 5, above the 15
pKa of phenolic acid, only 20% of the protonated form subsisted and these species were 16
adsorbed via coordinative bonding, without however any perceptible intercalation; (iii) and in 17
the regime with neutral environment (pH=7), the preponderance of GA anionic species led to 18
a poor adsorption which appeared to be only located at the external surface of the clay 19
mineral. 20
Keywords: Phenolic acids; Montmorillonite; Adsorption; Organic matter preservation.
21
1. Introduction
23
Clay minerals were recognized to stabilize soluble organic compounds through the 24
adsorption of dissolved OM in superficial horizons of soils (Gonzalez, 2002; Kögel-Knabner
25
et al., 2008; Schmidt et al., 2011; Kaiser et al., 2016). The confinement of organic compounds 26
within the interlayer space of clay minerals avoids any heterotrophic reactions and leads to 27
their preservation (Kaiser and Guggenberger, 2007; Wattel-Koekkoek and Buurman, 2004). It 28
also favors the generation of polymeric macromolecules via the condensation of single 29
monomers (Stevenson, 1982; Wang et al., 1983; Yariv and Cross, 2002). From the numerous 30
research works on the subject, and more particularly studies focusing on the understanding of 31
the interaction between clay minerals and humic substances leading to the formation of 32
humic-clay complexes, it appears that simple blocks of polymeric humic molecules or 33
elementary monomolecular compounds play a major role in the interaction with the mineral 34
surface and stabilization of the complexes (Greenland, 1971; Feng et al., 2005; Wang and
35
Xing, 2005; Chotzen et al., 2016; Chen et al., 2017). 36
The typology of physicochemical mechanisms allowing the establishment of perennial 37
association between organic compounds and mineral surfaces were extensively studied in 38
recent years. For example, ligand exchanges and cationic exchanges are reported as 39
sustainable organo-mineral interaction mechanisms (Keil and Mayer 2014; Lambert, 2018), 40
while low energy bonding such as van der Waals effects and hydrogen bonding are 41
acknowledged to establish easily reversible associations (Plante et al., 2005; Lutzow et al.,
42
2006). Aqueous media chemistry strongly constrains the preponderance of these mechanisms 43
(Arnarson and Keil, 2000). Background parameters as ionic strength and pH shape clays’ 44
state of charge as well as the degree of protonation of dissolved organic compounds. Hence, it 45
adsorption performances of humic compounds onto 2: 1 clay minerals at low pH and 47
suggested the increase of electrostatic phenomena to explain this observation. 48
Besides background properties, organic compounds intrinsic characteristics also 49
determine the extent of the adsorption and constrain the nature of preponderant bonding 50
mechanisms (Bu et al., 2017). In the presence of compounds with different spatial structures 51
and molecular functions, a competition to the occupation of adsorption sites can occur. For 52
example, the adsorption of amino acids onto montmorillonites is more effective than the 53
uptake of phenolic acids contained in the same aqueous mixture (Gao et al., 2017). 54
Understanding these mechanisms is mandatory to assess the potential use of clay minerals in 55
the adsorption of contaminants of various natures. Clays and clay minerals are investigated 56
raw, or after chemical modification, as an economically viable removal pathway of petroleum 57
by-products (Meleshyn and Tunega, 2011; Lamishane et al., 2016) and emerging 58
pharmaceutical molecules (Li et al., 2011; Thiebault et al., 2015; De Oliveira et al., 2017) 59
commonly encountered in hydrographic networks. The uptake performance may be boosted 60
via inorganic pillaring (Liu et al., 2015) or surfactant intercalation (De Oliveira and Guégan
61
2016). 62
Thus, the study of the fundamental mechanisms of organo-mineral aggregation has 63
applications in various fields. In this paper, the emphasis is placed on the organo-clay 64
association, and its further protective role. This role has been assessed in sedimentary 65
environments through different approaches (Kennedy and Wagner, 2011; Arndt and
66
Jorgensen, 2013; Mahamat Ahmat et al., 2016, 2017). It appeared that organo-clay 67
associations involving ionic exchanges and those resulting from interlayer intercalation 68
allowed efficient isolation against microbial stresses. In pedological context, the mechanism 69
has mainly been studied from the perspective of complex polymeric humic acids (Chen et al.,
70
While the main interaction mechanisms ensuring the stability of the organo-mineral 72
complexes are rather difficult to determine in the case of humic substances (Tombácz et al.,
73
2004; Chotzen et al., 2016; Chen et al., 2017), this research work aims at characterizing 74
precisely the main driving force leading to the aggregation of OM. For this purpose, we focus 75
on a simple carboxylic acid (gallic acid) as simple blocks or monomeric components in 76
polymeric humic acids. Gallic Acid (GA) is a phenolic acid found in vascularized plants, 77
inputted in soil horizons as single molecules or apart of large polyphenolic macromolecules 78
such as tannins and various ligno-cellulosic by-products. Here, in this research work, we 79
focus on the sorption mechanisms of GA onto montmorillonite under different experimental 80
conditions. 81
2. Materials and methods
82
2.1. Interaction of the carboxylic acids with a clay mineral
83
A natural Wyoming Na-montmorillonite (Na-Mt) was supplied by the Source Clay 84
Minerals Repository of the Clay Minerals Society. Its structural formula can be expressed as: 85
(Ca0.12Na0.32K0.05) [Al3.01Fe(III)0.41Mn0.01Mg0.54Ti0.02] [Si7,98Al0.02] O20(OH)4. Gallic Acid 86
(GA) was provided from Sigma Aldrich at 97.5 % purity grade and was used without further 87
treatment. This weak acid can be seen as a phenolic compound owing a carboxylic function 88
with 2 alcohol groups (Fig. 1). Its physico-chemical parameters (topological surface, pKa) are 89
summarized in Table 1. 90
The interactions between GA and Na-Mt were conducted in different experimental 91
conditions under various pH conditions: 2, 5 and 7. Typically, the adsorbent mass was 150 mg 92
in 50 mL solution spiked with a GA concentration between 10 mg. L-1 and 2 g. L-1. The pH 93
value was adjusted with both NaOH and HCl solutions (0.1 M). The solutions were stirred at 94
separated from the liquid one through a centrifugation step (5000 rpm; 10 min) and then 97
lyophilized before flash pyrolysis analyses. 98
2.2. Experimental Techniques
99
Adsorbed organic carbon was measured using flash pyrolysis (Thermo Scientific Flash 100
2000 organic analyzer) performed on organo-clay complexes in powder form. 101
The samples were also characterized via Fourier Transform Infrared Spectroscopy 102
(FT/IR) in the range 650-4000 cm-1. Measurements were realized using a Thermo Nicolet 103
6700 FT spectrometer equipped with a Deuterated Triglycine Sulfate (DTGS) detector at 104
room temperature and for different temperatures controlled by a Linkam thermal device 105
allowing us to characterize the thermal behavior on a wide range of temperature: 50-550°C. 106
The analyses were performed in transmission mode and each spectrum corresponded to the 107
average of 256 scans collected at 2 cm-1 ofresolution. 108
The d001 spacing’s of the organo-mineral complexes was determined by the first 00l 109
reflection from the X-rays patterns which were recorded on a conventional θ-θ Bragg-110
Brentano configuration by using a Thermo Electron ARL'XTRA diffractometer equipped with 111
a Cu anode (CuKα = 1.5418 Å) coupled with a Si(Li) solid detector. The diffractograms on 112
dry samples (100°C for 24 h) were performed between 1 and 24° (2θ) with an angular and 113
time steps of 0.04° and 10 s, respectively. 114
Thermal gravimetric analyses were carried out under atmospheric conditions at the 115
heating rate of 10 °C min−1 from room temperature (25°C) to 800 °C using the thermal 116
gravimetric analyzer (Model: STA PT 1600, manufactured by Linseis Company, Germany). 117
2.3. Sorption Modeling
118
The fitting of the resulting adsorption isotherms by using Langmuir, Freundlich and Dubinin– 119
Radushkevich (DR) equation models drive to numerous thermodynamic parameters allowing 120
that the whole organic molecules are adsorbed on singularized sites on the accessible surface 122
of the adsorbent, and each site hosts a unique molecule. This Langmuir model is expressed by 123
the following equation (LeVan and Vermeulen, 1981): 124
qe = qmax KL /[1 + (KL Ce )] (1)
125
where qe is adsorbed amount when equilibrium is reached (mol g-1); Ce is the
126
remaining concentration in the solution at equilibrium (mol L-1); qmax is the maximum
127
sorption capacity of the Na-Mt, and KL is the Langmuir constant (L mol-1) which is related to
128
Gibbs free energy ΔG° (kJ mol-1) through the thermodynamic equation (2): 129
ΔG° = -RTln KL (2)
130
where R represents the universal gas constant (8.314 J mol-1 K-1) and T the temperature 131
(K). 132
Freundlich and D-R equations takes into account surface heterogeneities on the 133
adsorption process and deal with the variabilities in the interaction mechanisms leading to the 134
adsorption of organic compounds that can form or be organized in multi-layers. Freundlich 135
adsorption model is a linear relation (LeVan and Vermeulen, 1981; Özcan et al., 2005) 136
expressed through the following equation: 137
ln qe = ln KF + 1/n (ln Ce) (3)
138
Where KF (g L-1) and n are constants and indicate respectively the extent of the
139
adsorption and the degree of non-linearity between GA and the smectite. Indeed, when the 140
term 1/n ranges between 0.1 and 1, it suggests that the adsorption mechanism is favorable 141
(Liu et al., 2011). D-R isotherms allow one to acquire complementary thermodynamic 142
parameters. Its equation is written as: 143
ln qe = ln qm + β*ε2 (4)
Where ε is the Polanyi potential, computed through the relation (5) 145
ε = RT ln (1+1/Ce) (5)
146
qm is the theoretical potential saturation capacity of the sorbent and β is the constant related to
147
the activity (mol2 J-2) connected to the mean free energy E of adsorption (kJ mol-1) via the 148
equation (6): 149
E = 1 / √2β (6)
150
This later parameter gives information whether the adsorption mechanism involves a cation 151
exchange or physical adsorption. Indeed, if the magnitude of E is below 8 kJ mol-1, 152
physisorption is envisaged, while for E > 8 kJ mol-1 the adsorption process follows an ion 153
exchange or a chemisorption mechanism. 154
Moreover, we used an error function (Ferror) in order to evaluate which equation model
155
was best suited to describe these processes. A lower result from the error function indicated a 156
smaller difference between adsorption capacity calculated by the model (qi cal) and the
157
experimental (qiexp). Ferror can be expressed according to the following Eq. (7)
158
Ferror = ∑ (qical – qiexp / qiexp)2 (7)
159
Where qi cal is a value of q predicted by the fitted model; qi exp is a value of q measured
160
experimentally; i indicates the values of the initial GA concentration of the experiments; and 161
P is the number of experiments performed.
162
3. Results and discussions
163
3.1. pH dependence on the adsorption of GA onto Na-Mt
164
The adsorption isotherms at the equilibrium stress out the actual affinity of GA with Na-165
when the equilibrium concentration increases. The slope of this growth attenuates at high 167
starting concentrations emphasizing a saturation state, excepted for the isotherm realized at 168
pH=5. 169
GA adsorption isotherms are properly fitted by the three equation models used as r2 values 170
display values between 0.95 and 0.99 and Ferror values are between 0.0010 and 0.130 for GA
171
(Table 2). Based on r2 values, experimental data seem to be better adjusted to the Langmuir 172
model, however its function errors are higher than 0.1 and to those for both Freundlich and 173
DR equations, which spread out from 0.001 to 0.004. This is a side effect of the logarithmic 174
scale adopted in Freundlich and DR representations. Although Langmuir equation properly 175
fitted experimental data, the two latter equations appear to be more suitable for modeling the 176
adsorption of the phenolic acid onto the clay mineral surface. Indeed, clay mineral shows a 177
heterogeneous surface leading to a distribution of several adsorption sites that are taken into 178
account in both Freundlich and DR equation models.
179
Under low pH conditions (i.e. below the pKa of GA), the adsorption is particularly enhanced, 180
with the predominance of the protonated form of GA (i.e. acidic form) in solution, due to the 181
decrease of the repulsion between the neutral GA and Na-Mt. However, the situation may be 182
rather twisted with antagonist effects. Indeed, under such pH conditions (pH=2), the pH value 183
is lower than the pH of zero net proton charge (pHZNPC) of Na-Mt, estimated about 4.5 184
(Tombàcz and Szekeres, 2006).For pH < pHZNPC, the electric charge of the edge-sites of clay 185
mineral surface changes from anionic to cationic, favoring the adsorption of phenolic acids in 186
their anionic form, (i.e. base) allowing a possible ion exchange process which enhances the 187
amount of adsorbed organic acids. In contrast, at pH=5 and 7, since both the clay mineral 188
edge-sites and the equilibrium ratio between the acid/base forms change (Fig. 3), the 189
adsorption isotherms while reducing the adsorbed amounts. This lowering is due to the 191
repulsion between anionic charges of both GA and Na-Mt. 192
193
Fig. 4 shows the X-ray diffraction patterns evolution of the GA-clay complexes (dried 194
for 48 hours at 100°C to prevent any water molecules in the interlayer space that may 195
interfere in the interpretation and understanding of a possible intercalation of the GA) 196
following the starting GA concentration. The diffraction patterns display several diffraction 197
peaks located between 5° and 9° (2θ) related to the 00l reflections. At pH=2, the 00l reflection 198
shifts to lower angle values, attesting the effective intercalation of the phenolic compound. As 199
well, it is interesting to remark the narrowing of these reflections suggesting an enhancement 200
of the organization in the layered material probably due to the increase of the GA density 201
within the clay–GA complexes. The basal spacing of GA-Mt complexes (i.e. d001), estimated 202
with the angular position of the 00l reflection, increases from 9.6 Å for a dehydrated clay 203
mineral to about 13.3 Å at pH=2. While an intercalation occurs at low pH, this phenomenon is 204
not observed for the other two pH, where a slight increase of the interlayer space to a value of 205
11 Å can be noticed (Fig. 5). This is probably due to both the clay mineral charge density and 206
the preponderance of the neutral (RCO2H) form which play an important role in the 207
adsorption as well its associated interaction mechanism (e.g. coordinative bonding ). 208
The contribution of FT/IR spectroscopy gives important information in the 209
characterization of GA-clay complexes derived from GA and Mt and confirms its actual 210
adsorption. Absorption bands observed at 1350, 1384 and 1470 cm-1 are assigned to stretching 211
modes of C – O. and bending modes of C-H of O-H respectively (Fig. 6). The strong vibration 212
at 1700 cm-1 was assigned to C=O stretching of GA’s carboxylic function. Besides the 213
existence of a drift or a temperature gradient in temperature during the FT/IR experiments, the 214
GA which is decomposed at about 300°C estimated through thermal gravimetric 216
measurements (Fig. 8). This last observation about the preservation of GA at high temperature 217
is related to its confinement within the interlayer space of Mt obtained for GA at pH=2. 218
TG analyses allow assessing the loss of weight of the adsorbent during a gradual 219
heating. Organo-clay minerals display usually three main weight losses during the heating. 220
The first one is associated to the evaporation of free and adsorbed water, ranging between the 221
initial temperature and 150°C. The second one is related to the thermal oxidation of adsorbed 222
organic compounds between 150 and 600°C with a maximal decomposition temperature 223
related to the characteristics of organic moieties. Finally, for temperatures higher than 600°C, 224
only the dehydroxylation of clay minerals is expected (Xie et al., 2001). Fig. 8 gathers TG and 225
DTG curves of GA-Na-Mt composites. Weight losses associated to the dehydration remains 226
independent from pH values. Corresponding DTG peaks occur between 98 and 105°C. With 227
growing temperature, the distinction between each sample is more pronounced. Hence, 228
between 150 and 600°C, no significant weight loss is displayed at pH=7, whereas noteworthy 229
weight losses are observed at the lower pH values (2 and 5). The first decomposition 230
temperature is noticed at 247°C and corresponds to the decomposition of acidic group of GA 231
(Rao et al., 1981; Hussein et al., 2009). In the organic matter combustion range, GA-Na-Mt 232
complexes aggregated at pH = 5 display one loss of weight (i.e. 340°C), while those formed at 233
pH=2 exhibit two noticeable losses at 310 and 410°C (Fig. 8). This splitting after interaction 234
at pH=2 might be explained by the distinction between GA adsorbed onto the external surface 235
of the clay mineral (i.e. lower temperature) and intercalated GA (i.e. higher temperature) (Zhu
236
et al., 2017). Hence, this result appears to be consistent with XRD results, in which an 237
intercalation of GA only occurs after interaction at pH=2. 238
3.2. Adsorption mechanisms and geochemical model of GA-Mt complexes
The observation that GA adsorption is higher at pH < pKa is in agreement with the 240
results of Rabiei et al., 2016 which stressed out higher GA adsorptions at pH=3 (33%) 241
compared to their experiments performed at pH=7 (20%). This pH-dependency is related to 242
the protonation capacity of GA’s polar appendage (COOH / COO-). It must be noted however, 243
that the pH control of GA adsorption cannot be generalized to the interaction of other phenol-244
based molecules. The study by Dolaksis et al., 2018 for instance, suggested that the 245
adsorption of phenolic cores devoid of polarizing COOH appendix is pH–independent. They 246
showed that low energy mechanisms (hydrogen bridges) drive the adsorption of 247
chlorophenols and nitrophenols onto silicate surfaces. 248
At a pH < pKa, GA is mainly protonated (neutral) and this form represents about 99.6 % of 249
the chemical species at pH=2 (Fig. 3). However, in acidic condition, the charge density of the 250
edge-sites of Na-Mt changes and switches below the pH of zero net proton charge estimated 251
at 4.5 (Table 1). Here, despite the possibility of an alteration of the structure and probably the 252
chemical composition of the layered material, the adsorption of GA appears to be enhanced in 253
acidic conditions. The parameters derived from the fitting procedure, and more particularly 254
those of the D-R model giving a free energy of adsorption E slightly above 8 kJ mol-1, 255
underlining the possibility of chemical process for adsorption or at least strong electrostatic 256
interactions. Although being less known and implied in the adsorption of anionic compounds 257
onto Mt, electrostatic interaction was recognized as the main driving force leading to the 258
intercalation of various kind of anionic compounds: tannic and benzoic acids, anionic 259
surfactants (Yan et al., 2007; Zhang et al., 2012; An and Dultz 2007), that may occur 260
nevertheless, in particular experimental conditions (low pH range) as it is the case here. 261
Moreover, the increase of the basal spacing of Na-Mt after the interaction with GA at pH=2 262
(i.e. + 3.7 Å) is consistent with the molecular size of GA (i.e. z=3.7 Å, Figure 1), emphasizing 263
The adsorption of anionic species, even weak, acts as a coating that may favor further 265
adsorption of organic molecules as organoclay materials do, nevertheless cannot only explain 266
the totality of the adsorbed amount reaching about 1.5 x 10-4 mol g-1. Indeed, GA is in anionic 267
form at a low concentration. The amount of anionic species may increase during the sorption 268
mechanism since the uptake involves the displacement of acid basic equilibrium. This may 269
allow in fine the adsorption of higher amount of anionic species. However, the effect of this 270
acid basic shift is limited. Protonated species of GA remain preponderant and are likely to be 271
adsorbed through other bonding mechanisms. Since GA is mainly neutral at pH < pKa as 272
explained before, its adsorption should be driven through physisorption mechanisms such as 273
molecular interaction (π- π interaction, van der Waals forces) with the prior adsorbed 274
molecules and by coordinative bonding through inorganic exchangeable cations located 275
within the interlayer space and with the carboxylic moieties as both FT / IR and XRD data 276
highlighted. A recent work stressed out the importance of ion-dipole interaction (a 277
coordinative bonding mechanism) as the main driving force for the adsorption of nonionic 278
surfactants (Guégan et al., 2017) onto a Mt surface and can be according to previous studies 279
in the literature (Sonon and Thompson, 2005; Deng et al., 2006) extended to nonionic 280
compounds and here GA in its neutral form (Fig. 7). 281
The preponderance and the role of this interaction is confirmed at a pH value of 5 where 282
the maximum adsorbed amounts at the equilibrium reaches 1.3 x 10-4 mol g-1. Based on the 283
diagram of preponderance of GA species in regards to pH, its neutral form represents about 284
20% at a pH=5 (Fig. 3). The experiment with the highest starting concentration of 2 g L-1 285
leads to 5.88 x 10-4 mol g-1 (if one does the hypothesis that such amount is adsorbed – number 286
of GA moles normalized to the mass of Mt introduced) where 20% are in the RCO2H form 287
(neutral one), thus driving to a value of 1.176 x 10-4 mol g-1, close to the experimental one 288
capacity as well as an anionic charge surface increasing the repulsion between adsorbent and 290
organic anions. Hence, the protonated form of GA is favored for adsorption, and may interact 291
to the inorganic exchangeable cations through coordinative bonding forces. However, it is 292
important to mention that 20% of the inorganic exchangeable cations are located onto the 293
external surface of the clay mineral (Swartzen-Allen and Matijević, 1975; Shainberg et al.,
294
1980; Patzko and Decany, 1993; Logdson and Laird, 2004), where they can be easily 295
mobilized for a cation-exchange or other interactions such as coordinative bonding or other 296
mechanism such as cationic bridges involving the anionic species and divalent compensating 297
cations. Here, the adsorption of GA does not lead to any intercalation as the XRD data 298
displayed and exclusively remains on the external surface. The presence of the inorganic 299
exchangeable cations at about 20% of the CEC on the external surface can be easily mobilized 300
through coordinative bonding or via complexation reactions (Fig. 7) with the phenolic acid of 301
which adsorbed amount match a value lesser than the 20% of the CEC (1.6 x 10-4 mol g-1). 302
Similar observations are noticed at pH=7, where the adsorbed amount of neutral GA is not 303
enough to lead to any intercalation as both XRD (Fig. 5) and FT/ IR data showed and restrict 304
at this pH range principally anionic species in the adsorption. Indeed, in contrast to the 305
previous pH, where the proportion of neutral species of GA represents about 96 and 20 % at 306
pH=2 and 5 respectively, at a pH=7, it is insignificant with only 0.25 % (Fig. 3). While, the 307
anionic form of GA is preponderant, an adsorption is surprisingly observed without however 308
any intercalation as it was the case for pH=5, leading to an adsorbed amount of about 1 x 10-4 309
mol g-1 (Fig. 2). Here, the adsorption may involve interaction mechanisms such as cationic 310
bridges between anionic GA and divalent inorganic cations. Hence, even mostly compensated 311
with Na+, around 20% of the compensating inorganic cations of Na-Mt are Ca2+, that are able 312
to sorb anionic species through cationic bridges (Errais et al., 2012; Thiebault et al., 2016;
313
pH = 5, although the distinction between these mechanisms is not possible based on these 315
experiments (Fig. 7). 316
3.3 Putting into perspective the behavior of GA with other phenolic-based compounds
317
and clay minerals
318
The adsorption of GA is enhanced at low pH, as polyphenolic humic molecules behave 319
(Feng et al., 2005; Zhang et al., 2012; Chotzen et al., 2016). Polyphenolic molecules as fulvic 320
and humic acids, exhibit indeed greater adsorption rates with growing acidity (Gouré-Douby
321
et al., 2018). For instance, the study of Chen et al. (2017) focusing on the adsorption of soil 322
macromolecular humic acids onto both montmorillonite and kaolinite, pointed out the 323
enhancement of the adsorption at low pH. GA adopts a similar behavior when background pH 324
ranges below its pKa and allows a preeminence of its protonated species. 325
In the case of natural polyphenolic molecules (humic substances), it has been repeatedly 326
observed that in addition to background acidity, the nature of the adsorbent has a predominant 327
role. Clay minerals of the 1:1 group such as kaolinite appear more conducive to the adsorption 328
of humic –type of polyphenols. This is promoted by edges electrostatic phenomena. The 329
extent of the adsorption through edge electrostatic interaction principally depends on the 330
physico-chemical properties and mineralogy of a clay mineral, and more particularly its pH of 331
zero net proton charge (pH ZNPC) which is estimated to 4.5 for Na-Mt (Tombàcz and Szekeres,
332
2006), lower than other soil clayey components such as kaolinite (Wiliams and Wiliams,
333
1978; Gupta and Miller, 2010). With its particular properties: a pHZNPC and background 334
protonation leading to a proper dispersion of clay mineral particles, kaolinite favors the 335
adsorption of macromolecular humic acids at large content in contrast to Na-Mt besides its 336
large CEC and specific surface area values (Chotzen et al., 2016). With similar background 337
be added to the key criteria ruling GA’s adsorption However, care should be taken in 340
comparing the uptake of these phenolic-based compounds as differences in molecular masses, 341
spatial arrangements and number of charges may induce differences in sorptive behaviors. 342
Although being poorer from quantitative perspective, the interaction of GA with Na-Mt 343
remains interesting from a protective point of view since its neutral form may intercalate (Fig.
344
2; Fig. 7) under specific acidic conditions (pH < pKa). Interlayers isolations are reported to 345
reduce bioavailability (Theng et al., 2001) and help to prevent biotic redox transformations. 346
In our case, the pH dependence of the adsorption appears to be consistent with the 347
trends shown by other phenolic and polyphenolic molecules. However, the nature of the 348
interactional mechanisms varies according to the molecules and does not display any 349
systematic pattern. Although our results suggest intercalation and surface complexations via 350
weak mechanisms (coordinative bonding), ligand exchanges are often involved in the 351
interaction of long phenolic chains. 352
This encourages to complexify our view of the aggregation modalities of clays and 353
phenol-like molecules and the possibility of organic matter stabilization process that it 354
inducts. Na-Mt interactional mechanisms seems to differ whether the involved organic 355
molecule are heavy macromolecular acids or singularized phenolic compounds. Thus, in a 356
pedological environment where micro-fauna regime induces high rates of polymeric lyses and 357
solubilizes a high quantity of phenolic acids, the stabilization process follows a different 358
pathway from the configuration where polymeric humic acids are weakly degraded. In the 359
first case, our data suggest that intercalation should prevail during the adsorption of poly-360
phenolic acids by-products in acidic pH conditions, while several studies point out adsorption 361
on the edge and ligand exchanges in the ecological context where polymeric forms are 362
4. Conclusions
364
The series of organo-clay interactions performed here under evolutionary batch 365
equilibrium conditions allowed to characterize the adsorption mechanisms of GA, a common 366
phenolic acid in natural pedological media, onto Na-Mt. The parameters derived from fitting 367
of GA data with a reasonable agreement (high r2 values) to the adsorption models used: 368
Langmuir, Freundlich and D-R equations, gave pertinent insights for a proper description of 369
the phenomena under different approaches which pointed out the good affinity of the phenolic 370
acid to Na-Mt. Additional experimental results obtained by FT/IR, TGA and XRD 371
corroborated the actual adsorption of GA onto Na-Mt. 372
From the set of data, it appears that this monomolecular acid is mainly adsorbed 373
through coordinative bonding interaction in contrast to macromolecular humic acids at low 374
pH range, where ligand exchanges or complexation reaction occur according to the literature. 375
Here, GA, a singularized element of large pedological compounds, interacts with the clayey 376
mineral mainly via both surface and interlayer processes at low pH, leading to its confinement 377
within the interlayer space which may allow a sustainable preservation of the organic matter 378
in that way. 379
Acknowledgement
380
This study was supported by the project MONITOPOL funded by the French region 381
Centre Val de Loire (grant number 00117247). The authors are grateful to Marielle Hatton for 382
her analytical contribution. 383
Figures Captions
384
Fig. 2: Adsorption isotherms of Gallic Acid. Beige squares represent the data obtained at pH 386
2, green ones are for the data measured at pH 5 and the red triangles represent those collected 387
at pH 5. The continuous line represents Langmuir model fit. 388
Fig. 3: GA speciation following pH conditions. 389
Fig. 4: Graph of the 3D evolution of XRD diffraction patterns of dehydrated Gallic Acid (GA) 390
and Na-Mt composite samples as a function of the starting GA concentrations in solution for 391
pH=2. Only the results going up to 0.1 g L-1 arepresented here, since no particular evolution 392
of (001) planes was observed beyond this concentration. 393
Fig. 5: Evolution of the d001 basal spacing determined by the 00l reflection of Na-Mt layers 394
obtained for GA / Na-Mt humic-clay like samples. 395
Fig. 6: FT/IR spectra of GA/Na-Mt composite samples (pH=2): thermic evolution. 396
Fig.7: Schematic representation of the possible adsorption mechanisms leading to the 397
intercalation of GA within the interlayer of a Na-Mt at pH=2, and adsorption for the pH=5 398
and 7. 399
Fig. 8: TG (solid lines) and DTG (dashed lines) curves of GA/Na-Mt (CGA = 0.1 g L-1) 400
composite samples after interaction at pH=2 (dark gray lines); pH=5 (dark lines) and pH=7 401
(light gray lines). 402
Tables Captions
403
Table 1: Physico-chemical properties of implemented clay mineral (Na-Mt) and GA. 404
Table 2: Adsorption isotherm constants determined with Langmuir, Freundlich, and Dubinin-405
Radushkevich model fit for the adsorption of GA onto Na-Mt for different pH. 406
Supplementary data
Appendix 1: Spectroscopic observations (FT/IR) of GA-Na-Mt complexes aggregated at 408
different levels of GA starting concentrations. 409
References
410
An, J-H., Dultz, S., 2007. Adsorption of tannic acid on chitosan-montmorillonite as a function 411
of pH and surface charge properties. Applied Clay Science 36, 256-264. 412
Arnarson, T.S., Keil, R.G., 2000. Mechanisms of pore water organic matter adsorption to 413
montmorillonite. Marine Chemistry 71, 309-320. 414
Arndt, S., Jorgensen, B.B., 2013. Quantifying the degradation of organic matter in marine 415
sediments: A review and synthesis. Earth-Science Reviews 123, 53-86. 416
Bu, H., Yuan, P., Liu, H., Liu, D., Liu, J., He, H., Li, Z. (2017). Effects of complexation 417
between organic matter (OM) and clay mineral on OM pyrolysis. Geochimica et 418
Cosmochimica Acta, 212, 1-15. 419
Chen, H., Koopal, L.K., Xiong, L.K., Xiong, J., Avena, M., Tan, W., 2017. Mechanisms of 420
soil humic acid adsorption onto montmorillonite and kaolinite. Journal of Colloid and 421
Interface Science 504, 457-467. 422
Chotzen, A.R., Polubesova, T., Chefetz, B., Mishael, Y.G., 2016. Adsorption of soil-derived 423
humic acid by seven clay minerals, a systematic study. Clays and Clay Minerals 64, 424
628-638. 425
De Oliveira, T., Guegan, R., 2016. Coupled Organoclay / Micelle Action for the Adsorption 426
of Diclofenac. Environmental Science & Technology 50, 10209-10215 427
De Oliveira, T., Guégan, R., Thiebault, T., Le Milbeau C., Muller F., Teixeira, V., Giovanela, 428
M., Boussafir, M., 2017. Adsorption of Diclofenac onto organoclays: effect of 429
surfactant and environmental (pH and temperature) conditions. Journal of Hazardous 430
Deng, Y., Dixon, J.B., Norman White, G., 2006. Bonding mechanisms and conformation of 432
poly(ethylene oxide)-based surfactants in interlayer of smectite. Colloid and Polymer 433
Science 284, 347-356. 434
Derakhshani, E., Naghizadeh, A., 2018. Optimization of humic acid removal by adsorption 435
onto bentonite and montmorillonite nanoparticles. Journal of Molecular Liquids 259, 436
76-81. 437
Dolaksiz, E.Y., Temel, F., Tabakci, M., 2018. Adsorption of phenolic compounds onto 438
calix[4]arene-bonded silica gels from aqueous solutions. Reactive and Functional 439
Polymers 126, 27 - 35. 440
Errais, E., Duplay, J., Elhabiri,M., Khodja, M., Ocampo, R., Baltenweck-Guyot, R., 441
Darragi,F., 2012. Anionic RR120 dye adsorption onto raw clay: Surface properties and 442
adsorption mechanism. Colloids and Surfaces A: Physicochemical and Engineering 443
Aspects 403, 69 - 78. 444
Feng, X., Simpson, A.J., Simpson, M.J., 2005. Chemical and mineralogical controls on humic 445
acid sorption to clay mineral surfaces. Organic Geochemistry 36, 1553-1566. 446
Gao, J., Jansen, B., Cerli, B., Helmus, R., Mikutta, R., Dultz, S., Guggenberger, G., Kalbitz, 447
K., 2017. Competition and surface conditioning alter the adsorption of phenolic and 448
amino acids on soil minerals. European Journal of Soil Science 68, 667-677. 449
Gonzalez, J- M., 2002. Role of clay minerals on soil organic matter and humification. Ph.D 450
thesis of Iowa State University, 212 pp. 451
Gouré-Douby, H., Martias, C., Smith, A., Villandier, N., Sol, V., Gloaguen, V., Feuillade, G., 452
2018. Adsorption of fulvic and humic like acids on surfaces of clays: Relation with 453
Guégan, R., Veron, E., Le Forestier, L., Ogawa, M., Cadars, S., 2017 Structure and dynamics 455
of nonionic surfactant aggregates in layered materials. Langmuir 33, 9759-9771. 456
Gupta, V., Miller, J.D., 2010. Surface force measurements at the basal planes of ordered 457
kaolinite particles. Journal of Colloid and Interface Science 344, 362-371. 458
Greenland, D.J., 1971. Interaction between humic and fulvic acids and clays. Soil Science, 459
111 34-41. 460
Hussein, M.Z.B., Ghotbi, M.Y., Yahaya, A.H., Abd Rahman, M.Z., 2009. Synthesis and 461
characterization of (zinc-layered-gallate) nanohybrid using structural memory effect. 462
Materials Chemistry and Physics 113, 491-496. 463
Kaiser, K., Guggenberger, G., 2007. Sorptive stabilization of organic matter by microporous 464
goethite: Sorption into small pores vs. surface complexation. European Journal of Soil 465
Science 58, 45-59. 466
Kaiser, K., Zederer, D.P., Ellerbrock, R.H., Sommer, M., Ludwig, B., 2016. Effects of 467
mineral characteristics on content, composition, and stability of organic matter fractions 468
separated from seven forest topsoils of different pedogenesis. Geoderma 263, 1-7. 469
Keil, R.G., Mayer, L.M., 2014. Mineral matrices and organic matter. Treatise on 470
Geochemistry (Second Edition) 12, 337-359. 471
Kennedy, M.J., Wagner, T., 2011. Clay mineral continental amplifier for marine carbon 472
sequestration in a greenhouse ocean. Proceedings of the National Academy of Sciences 473
of the United States of America 108, 9776-9781. 474
Kögel-Knabner, I., Guggenberger, G., Kleber, M., 2008. Organo-mineral associations in 475
temperate soils: Integrating biology, mineralogy, and organic matter chemistry. Journal 476
of Plant Nutrition and Soil Science-Zeitschrift für Pflanzenernahrung und Bodenkunde 477
Lambert, J.F., 2018. Organic pollutants adsorption on clay minerals. Developments in Clay 479
Science 9, 195-253. 480
Lamishhane, S., Bal Krishna;K.C., Sarukkalige, R., 2016. Polycylic Aromatic Hyrdrocarbons 481
(PAHs) removal by sorption: A review. Chemosphere 148, 336-353. 482
LeVan, M.D., Vermeulen,T., 1981. Binary Langmuir and Freundlich isotherms for ideal 483
adsorbed solutions. The Journal of Physical Chemistry 85, 3247-3250. 484
Li, Z., Chang, P-H., Jiang. W-T., Jean, J-S., Hong, H., Liao, L., 2011. Removal of 485
diphenhydramine from water by swelling clay minerals. Journal of Colloid and 486
Interface Science 360, 227-232. 487
Liu, R., Zhang, B., Mei, D., Zhang, H., Liu, J., 2011. Adsorption of methyl violet from 488
aqueous solution by halloysite nanotubes. Desalination 268, 111-116. 489
Liu, Y., Dong, C., Wei, W., Yuan, W., Li, K., 2015. Adsorption of levofloxacin onto an iron-490
pillared montmorillonite (clay mineral): Kinetics, equilibrium and mechanism. Applied 491
Clay Science 118, 301-307. 492
Logsdon, S.D., Laird, D.A., 2004. Electrical conductivity spectra spectra of smectites as 493
influenced by saturating cation and humidity. Clays and Clay Minerals 52, 411 – 420. 494
Lutzow, M., Kögel-Knabner, I., Ekschmitt, K., Matzner, E., Guggenberger, G., Marschner, 495
B., Flessa, H., 2006. Stabilization of organic matter in temperate soils: Mechanisms and 496
their relevance under different soil conditions-A review. European Journal of Soil 497
Science 57, 426-445. 498
Mahamat-Ahmat, A., Boussafir, M., Le Milbeau, C., Guégan, R., Valdès, J, Guiñez, M., 499
Sifeddine, A., Le Forestier, L., 2016. Organic matter-clay interaction along a seawater 500
column of the Eastern Pacific upwelling system (Antofagasta bay, Chile): implications 501
Mahamat-Ahmat, A., Boussafir, M., Le Milbeau, Guégan, R., De Oliveira, T., Le Forestier, 503
L., 2017. Organic matter and clay interaction in a meromictic lake: implications for 504
source rock OM preservation (Lac Pavin, Puy-de-Dôme, France). Organic 505
Geochemistry 109, 47-57. 506
Meleshyn, A., Tunega., 2011. Adsorption of phenanthrene on Na-Montmorillonite: A model 507
study. Geoderma 169, 41-46. 508
Nam Jang, B., Wilkie, C.A., 2004. A TGA/FTIR and mass spectral study on the thermal 509
degradation of bisphenol A polycarbonate. Polymer Degradation and Stability 86, 419-510
430. 511
Özcan, A.S., Erdem, B., Özcan, A., 2005. Adsorption of acid blue 193 from aqueous solutions 512
onto BTMA-bentonite. Colloids and Surfaces A: Physicochemical and Engineering 513
Aspects 266, 73-81. 514
Patzko., A., Decany., I., 1993 Ion exchange and molecular adsorption of a cationic surfactant 515
on clay minerals . Colloids and Surfaces A: Physicochemical and Engineering Aspects. 516
71 (1993) 299-307. 517
Plante, A.F., Pernes, M., Chenu, C., 2005. Changes in clay-associated organic matter quality 518
in a C depletion sequence as measured by differential thermal analyses. Geoderma 129, 519
186-199. 520
Rabiei, M., Sabahi, H., Rezayan, A.H., 2016. Gallic acid-loaded montmorillonite 521
nanostructure as a new controlled release system. Applied Clay Science 119, 236-242. 522
Rao, C.R.M., Reddy, V.B., Mehrotra, P.N., 1981. Derivatographic studies of gallic acid. 523
Thermochimica Acta 46, 65-69.Shainberg, I., Oster, J.D., Wood, J.D., 1980. Sodium / 524
Calcium Exchange in Montmorillonite and Illite Suspensions 1, Soil Science Society of 525
Swartzen-Allen, S.L., Matijević, E., 1975. Colloid and surface properties of clay suspensions: 527
II. Electrophoresis and cation adsorption of montmorillonite. Journal of Colloid 528
Interface Science 50, 143–153. 529
Sonon, L.S., Thompson, M.L., 2005. Sorption of a nonionic polyoxyethylene lauryl ether 530
surfactant by 2:1 layer silicates. Clays and Clay minerals 53, 45-54. 531
Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., 532
Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, 533
D.P., Weiner, S., Trumbore, S.E., 2011. Persistence of soil organic matter as an 534
ecosystem property. Nature 478, 49-56. 535
Stevenson, F.J. 1982. Humus Chemistry. John Wiley & Sons, New York. 536
Theng, B.K.G., Aslabie, J., Fraser, R., 2001. Bioavailability of phenanthrene intercalated into 537
an alkylamonium-montmorillonite clay. Soil Biology and Biogeochemistry 33, 845-848. 538
Thiebault, T., Guégan, R., Boussafir, M., 2015. Adsorption mechanisms of emerging micro-539
pollutants with a clay mineral: case of tramadol and doxepine pharmaceutical products. 540
Journal of Colloid and Interface Science 453, 1-8. 541
Thiebault, T., Boussafir, M., Guégan, R., Le Milbeau, C., Le Forestier, L., 2016. Clayey-sand 542
filter for the removal of pharmaceuticals from wastewater effluent: percolation 543
experiments. Environmental Science Water Research & Technology 2, 529-538. 544
Tombácz, E., Libor, Z., Illés, E., Majzik, A., Klumpp, E., 2004. The role of reactive surface 545
sites and complexation by humic acids in the interaction of clay mineral and iron oxide 546
particles. Organic Geochemistry 35, 257-267. 547
Tombácz, E., Szekeres, M., 2006. Surface charge heterogeneity of kaolinite in aqueous 548
Wang, K., Xing., B., 2005. Structural and sorption characteristics of adsorbed humic acid on 550
clay minerals. Journal of Environmental Quality, 34 342-349. 551
Wang, T.S.C., Wang, M.C., Ferng, Y.L., 1983. Catalytic synthesis of humic substances by 552
natural clays, silts, and soils. Soil Science 135, 350-360. 553
Wattel-Koekkoek, E.J.W., Buurman, P., 2004. Mean residence time of kaolinite and smectite-554
bound organic matter in Mozambiquan soils. Soil Science Society of America Journal 555
68, 154-161. 556
Williams, D.J.A., Williams, K.P., 1978. Electrophoresis and zeta potential of kaolinite. 557
Journal of Colloid and Interface Science 65, 79-87. 558
Xie, W., Gao, Z., Liu, K., Pan, W.-P., Vaia, R., Hunter, D., Singh, A., 2001. Thermal 559
characterization of organically modified montmorillonite. Thermochimica Acta 367-560
368, 339-350. 561
Yan, L., Wang, J., Yu, H., Wei, Q., Du, B., Shan, X., 2007. Adsorption of benzoic acid by 562
CTAB exchanged montmorillonite. Applied Clay Science 37, 226-230. 563
Yariv, S., Cross, H., 2002. Organo-clay complexes and interactions. Marcel Dekker, New 564
York 688pp. 565
Zhang, L., Luo, L., Zhang, S., 2012. Integrated investigations on the adsorption mechanisms 566
of fulvic and humic acids on three clay minerals. Colloids and Surfaces A: 567
Physicochemical and Engineering Aspects 406, 84-90. 568
Zhu, J., Zhang, P., Qing, Y., Wen, K., Su, X., Ma, L., Wei, J., Liu, H., He, H., Xi, Y., 2017. 569
Novel intercalation mechanism of zwitterionic surfactant modified montmorillonites. 570
Applied Clay Science 141, 265- 271. 571
572
Highlights
- Gallic acid (GA) adsorption onto Na-Mt was performed in batch conditions; - Data were fitted to Dubinin-Radushkevitch, Langmuir and Freundlich equations; - Following the acidity, both edge and interlayer interactions modes were observed; - GA intercalates under its neutral form;
Properties Values
Na-MMt C.E.C (mol.g-1) 8.10-4
pznc 4.5
GA COOH pKa 4.4
Surface area (Å2) 98
Table 1
x=7.64 Å
y=5.87 Å
z=3.72 Å
Figure 1
Figure 4
Figure 6
Na
+Cla
y Mineral Na-Mt
d
001≈9.7Å
In
tercalation of GA
pH < pKa
d
001≈13.7Å
External adsorption of GA
pH > pKa
d
001≈9.7Å
0 200 400 600 800 1000
